Alternative transportation fuels have been represented as enablers to reduce toxic emissions in comparison to those generated by hydrocarbon fuels. At the same time, tighter emission standards and significant innovation in catalyst formulations and engine controls have led to dramatic improvements in the low emission performance and robustness of gasoline and diesel engine systems. This has certainly reduced the environmental differential between optimized and alternative fuel vehicle systems. However, many technical challenges remain to make the conventionally fueled internal combustion engine a nearly zero emission system having the efficiency necessary to make the vehicle commercially viable.
Alternative fuels cover a wide spectrum of potential environmental benefits, ranging from incremental toxic and carbon dioxide (CO2) emission improvements (reformulated gasoline, alcohols, liquefied propane gas, etc.) to significant toxic and CO2 emission improvements (natural gas, DME, etc.). Hydrogen is clearly the ultimate environmental fuel, with potential as a nearly emission free internal combustion engine fuel (including CO2 if it comes from a non-fossil source). Unfortunately, the market-based economics of alternative fuels, or new power train systems, are uncertain in the short to mid-term.
The automotive industry has made very significant progress in reducing automotive emissions in both the mandated test procedures and in the “real world”. This has resulted in some added cost and complexity of engine management systems, yet those costs are offset by other advantages of computer controls: increased power density, fuel efficiency, drivability, reliability and real-time diagnostics.
Future initiatives to require zero emission vehicles appear to be taking us into a new regulatory paradigm where asymptotically smaller environmental benefits come at larger incremental costs. Yet, even an “ultra low emission” certified vehicle may emit high emissions in limited extreme ambient and operating conditions or with failed or degraded components.
One approach to addressing the issue of emissions is the employment of fuel cells, particularly solid oxide fuel cells (“SOFC”), in an automobile. A fuel cell is an energy conversion device that generates electricity and heat by electrochemically combining a gaseous fuel, such as hydrogen, carbon monoxide, or a hydrocarbon, and an oxidant, such as air or oxygen, across an ion-conducting electrolyte. The fuel cell converts chemical energy into electrical energy. SOFCs are constructed entirely of solid-state materials, utilizing an ion conductive oxide ceramic as the electrolyte. An electrochemical cell in a SOFC may comprise an anode and a cathode with an electrolyte disposed therebetween. The oxidant passes over the oxygen electrode (cathode) while the fuel passes over the fuel electrode (anode), generating electricity, water, and heat.
In a SOFC, a fuel flows to the anode where it is oxidized by oxygen ions from the electrolyte, producing electrons that are released to an external circuit, and mostly water and carbon dioxide are removed in the fuel flow stream. At the cathode, the oxidant accepts electrons from the external circuit to form oxygen ions. The oxygen ions migrate across the electrolyte to the anode. The flow of electrons through the external circuit provides for consumable or storable electricity. However, each individual electrochemical cell generates a relatively small voltage. Higher voltages are attained by electrically connecting a plurality of electrochemical cells in series to form a stack.
A SOFC cell stack also includes conduits or manifolds to allow passage of the fuel and oxidant into the stack, and byproducts, excess fuel, and oxidant out of the stack. Generally, oxidant is fed to the structure from a manifold located on one side of the stack, while fuel is provided from a manifold located on an adjacent side of the stack. The fuel and oxidant are generally pumped through the manifolds and introduced to a flow field disposed adjacent to the appropriate electrode. The flow fields that direct the fuel and oxidant to the respective electrodes create oxidant and fuel flows across the electrodes that are perpendicular to one another.
Seals preferably are provided around the edges of the various cell stack components to inhibit crossover of fuel and/or oxidant. Seals may be disposed between electrodes and adjacent flow fields, around manifolds, between flow fields and cell separators, and elsewhere. One factor in establishing SOFC reliability is the integrity of these seals.
Leaks in manifold seals, electrochemical seals, or other defects may lead to the SOFC failure. Further, if molecular oxygen crosses over to the anode forming an oxidizing environment, anode oxidation may occur to create a chemical and volume change that may result in mechanical failure of the SOFC cell. Accordingly, flatness between components intended to form a seal may affect various aspects of solid oxide fuel cell systems.
As used herein, flatness is defined as having a horizontal surface without a slope, tilt, or curvature. Various phenomena may affect the flatness of a ceramic component. One of these phenomena is shrinkage mismatch between a porous nickel-yttria stabilized zirconia anode and a more dense yttria stabilized zirconia electrolyte. This may occur during sintering and may cause the component to become warped or to develop a camber (i.e., the condition of having or causing to have an arched surface). Shrinkage mismatch between two adjacent layers tends to manifest itself as edge curl in planar ceramic components. This problem also commonly leads to cracking due to internal stresses. Previous attempts directed to reduce or eliminate the degree of camber, or edge curl, formed when dissimilar materials are sintered together include the so-called creep flattening process in which the sintered cells are fired at high temperature under ceramic weights. This technique incorporates the use of flat plate (parts formed to a certain level of flatness, or machines to a flatness tolerance) that applies an equally distributed load across a heated ceramic planer cell or cell component. Upon approaching the sintering temperature of the ceramic part, flatness is achieved through high temperature creep in the material when maintained in this constrained environment over time. This method is used on parts that have already undergone the sintering process and are free of all combustible products, such as organics that have been added originally as processing aids. However, creep flattening is time consuming and requires large amounts of additional energy due to the firing times and the number of steps required. Also, creep flattening only reduces camber by a limited amount.
A second cause of camber in the sintered multilayer ceramic component is a mismatch in the coefficient of thermal expansion (CTE) between the two adjacent layers. This typically affects the shape of the whole ceramic planar multilayer part and is less significant at the edge. Upon cooling from the temperature required to co-sinter these layers, the two sintered layers contract at different rates due to differences in CTE. Due to the difference in inherent properties of the two materials joined to form the body, the mismatch could lead to significant camber. This camber cannot be removed via the creep flattening process. This camber can sometimes be reduced by design of the cell (i.e. one layer being much thicker than the other and dominate the contraction behavior).
Accordingly, there exists a need for flat ceramic cell components of solid oxide fuel cell systems, and a method of producing such components. In particular, a method to produce components in which the camber associated with mismatched shrinkage and CTE mismatch between fuel cell components during sintering and other aspects of fuel cell production is reduced or even eliminated. A need also exists for methods to produce flat ceramic cell components in a cost effective manner, requiring less energy than processes previously attempted.